i) Field of the Invention
This invention relates to a method of processing an interstitial free steel to increase strength and toughness of the steel; and to an interstitial free steel having an average grain size of up to 5 .mu.m, in particular ultra-fine grain sizes of 1 to 2 .mu.m, in particular such steels exhibit superior strength and toughness.
ii) Description of Prior Art
In steel, a high level of cold formability can be attained by reducing the concentration of interstitial elements, i.e, C and N, to a low level. Removal of these elements from the matrix is performed largely by vacuum degassing techniques. The resulting low interstitial concentration can be further reduced by the addition of Ti and/or Nb, which combine with C and N, leading to a solute level of these elements of only a few parts per million. These steels are known as interstitial free, or IF steels, and are, at present, mainly used in deep drawing applications.
It is well known that in polycrystalline metals grain size exhibits a strong effect on the mechanical properties; the finer the grain size the greater the strength or hardness, and the higher the toughness. Many attempts have been made to refine the ferrite grain size, because this is the only microstructural characteristic which can simultaneously improve both the yield strength and the toughness.
Yada et al. in U.S. Pat. No. 4,466,842 describe a technique for producing ultrafine grained ferrite in conventional C-Mn steels. According to their method, ultrafine grained ferrite is produced when such steels are rolled in the intercritical region, i.e., the austenite-plus-ferrite region, a two phase region between the single phase austenitic region and the single phase ferrite region. Yada et al. attribute this grain size refinement to the dynamic transformation of austenite to ferrite, as well as to the dynamic recrystallization of ferrite. It is probable that the former mechanism dominates, in which ultrafine grained ferrite is produced as a result of the repeated nucleation of ferrite at grain boundaries, with the dynamic recrystallization of ferrite playing only a minor role. Furthermore, Yada et al. specify that the dynamic recrystallization of ferrite only takes place in the intercritical region.